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          buuoj.cn上的CTF杂项练习题(一)
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<h1 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h1><p>CTF杂项练习，基本上做不太来,年纪大喽，无语呀。坐标<a href="https://buuoj.cn/challenges" target="_blank" rel="noopener">https://buuoj.cn/challenges</a></p>
<a id="more"></a>
<h1 id="开始做题"><a href="#开始做题" class="headerlink" title="开始做题"></a>开始做题</h1><h2 id="第一题-大白"><a href="#第一题-大白" class="headerlink" title="第一题 大白"></a>第一题 大白</h2><h3 id="审题"><a href="#审题" class="headerlink" title="审题"></a>审题</h3><p>看起来可爱的大白,其实一点也不可爱。<br><img src="daibai.png" alt="大白">根据CTF的套路这肯定不是一张正常的图片，应当是隐写题。</p>
<h3 id="尝试破解大白的秘密"><a href="#尝试破解大白的秘密" class="headerlink" title="尝试破解大白的秘密"></a>尝试破解大白的秘密</h3><h4 id="尝试一"><a href="#尝试一" class="headerlink" title="尝试一"></a>尝试一</h4><p>祭出查看隐藏文件的神器binwalk。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">$ binwalk dabai.png</span><br><span class="line"></span><br><span class="line">DECIMAL       HEXADECIMAL     DESCRIPTION</span><br><span class="line">--------------------------------------------------------------------------------</span><br><span class="line">0             0x0             PNG image, 679 x 256, 8-bit&#x2F;color RGBA, non-interlaced</span><br><span class="line">91            0x5B            Zlib compressed data, compressed</span><br></pre></td></tr></table></figure>
<p>看样子是好像是有附加一个什么文件，实际用foremost命令及binwalk -e 命令都未能成功的分离图片。看样子思路错了。 </p>
<h4 id="尝试二"><a href="#尝试二" class="headerlink" title="尝试二"></a>尝试二</h4><p>使用Stegsolve工具对图片进行通道，LSB等分析。基本没有啥有效信息。</p>
<h3 id="正式破解"><a href="#正式破解" class="headerlink" title="正式破解"></a>正式破解</h3><p>没啥好办法了，通过百度查了一下前辈的做法。发现我忽略了一个信息。这个图片在VScode下直接提示了错误，而且通过浏览器也无法正常打开这个图片。说明这个图片存在错误。根据网上的做法这个应当修改了文件导致CRC效验错误。</p>
<h4 id="PNG文件头知识。"><a href="#PNG文件头知识。" class="headerlink" title="PNG文件头知识。"></a>PNG文件头知识。</h4><p><img src="png-head.png" alt="PNG文件头"></p>
<h4 id="还原被修改的信息"><a href="#还原被修改的信息" class="headerlink" title="还原被修改的信息"></a>还原被修改的信息</h4><p>没啥好想的了，原始图片大小我也不知道呀！写了个小程序来跑CRC校验。上代码：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> sys</span><br><span class="line"><span class="keyword">import</span> re</span><br><span class="line"><span class="keyword">import</span> binascii</span><br><span class="line"><span class="keyword">import</span> struct</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">revpng</span><span class="params">(para1,para2)</span>:</span></span><br><span class="line">    <span class="comment">#随便读几个字节</span></span><br><span class="line">    <span class="comment">#lang = 100</span></span><br><span class="line">    <span class="comment"># 读取原zip文件</span></span><br><span class="line">    png = open(para1,<span class="string">'rb'</span>)</span><br><span class="line">    <span class="comment">#读取2进制</span></span><br><span class="line">    png_bin = png.read()</span><br><span class="line">    <span class="comment">#转成字符串方便比较</span></span><br><span class="line">    png_content= png_bin.hex().upper()</span><br><span class="line">    png.close()</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">    <span class="comment">#PNG文件头</span></span><br><span class="line">    png_head = <span class="string">r'89504E470D0A1A0A(.&#123;8&#125;)49484452'</span></span><br><span class="line">    <span class="comment">#看能否能匹配出数据区长度</span></span><br><span class="line">    match_contents = re.findall(png_head, png_content)</span><br><span class="line">    <span class="keyword">if</span> match_contents==[]:</span><br><span class="line">        print(<span class="string">"不是PNG文件退出\n"</span>)</span><br><span class="line">        <span class="keyword">return</span> </span><br><span class="line">    </span><br><span class="line">    <span class="comment">#得到PNG数据区长度</span></span><br><span class="line">    data_len=int(match_contents[<span class="number">0</span>],<span class="number">16</span>)</span><br><span class="line">    <span class="comment">#文件头长度</span></span><br><span class="line">    png_head_len=<span class="number">16</span>+data_len</span><br><span class="line">    <span class="comment">#png文件CRC32效验值，数据区后4位即为CRC32校验位</span></span><br><span class="line">    png_crc32_str=png_bin[png_head_len:png_head_len+<span class="number">4</span>].hex()</span><br><span class="line">    png_crc32=int(png_crc32_str,<span class="number">16</span>)</span><br><span class="line"></span><br><span class="line">    <span class="comment">#图片宽度</span></span><br><span class="line">    width=<span class="number">0</span></span><br><span class="line">    <span class="comment">#图片长度</span></span><br><span class="line">    height=<span class="number">0</span></span><br><span class="line">    <span class="comment">#循环查找文件的长宽高</span></span><br><span class="line">    <span class="keyword">for</span> i <span class="keyword">in</span> range(<span class="number">1024</span>):</span><br><span class="line">        data=png_bin[<span class="number">12</span>:<span class="number">16</span>]+struct.pack(<span class="string">'&gt;i'</span>,i)</span><br><span class="line">        <span class="keyword">for</span> j <span class="keyword">in</span> range(<span class="number">1024</span>):</span><br><span class="line">            data2=data+struct.pack(<span class="string">'&gt;i'</span>,j)+png_bin[<span class="number">24</span>:png_head_len]</span><br><span class="line">            crc32 = binascii.crc32(data2) &amp; <span class="number">0xffffffff</span></span><br><span class="line">            <span class="keyword">if</span> crc32 == png_crc32:</span><br><span class="line">                width=i</span><br><span class="line">                height=j</span><br><span class="line">                <span class="keyword">break</span></span><br><span class="line"></span><br><span class="line">    <span class="keyword">if</span> width!=<span class="number">0</span> <span class="keyword">and</span> height!=<span class="number">0</span>:</span><br><span class="line">        png_bin=png_bin[<span class="number">0</span>:<span class="number">16</span>]+struct.pack(<span class="string">'&gt;i'</span>,width)+struct.pack(<span class="string">'&gt;i'</span>,height)+png_bin[<span class="number">24</span>:<span class="number">-1</span>]</span><br><span class="line">        <span class="keyword">with</span> open(para2,<span class="string">'wb'</span>) <span class="keyword">as</span> f:</span><br><span class="line">            f.write(png_bin)</span><br><span class="line">        f.close()</span><br><span class="line">    <span class="keyword">else</span>:</span><br><span class="line">        print(<span class="string">'文件修复失败'</span>)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span> __name__ == <span class="string">'__main__'</span>:</span><br><span class="line">	<span class="keyword">if</span>(len(sys.argv) != <span class="number">3</span>):</span><br><span class="line">		print(<span class="string">'\n请参照如下命令'</span>)</span><br><span class="line">		print(<span class="string">' python 需要修复的文件.png 修复后的文件.png\n'</span>)</span><br><span class="line">	<span class="keyword">else</span>:</span><br><span class="line">		para = sys.argv</span><br><span class="line">		revpng(para[<span class="number">1</span>],para[<span class="number">2</span>])</span><br></pre></td></tr></table></figure>
<pre><code>注：以上程序在python3.8上调试通过。</code></pre><h4 id="得到flag"><a href="#得到flag" class="headerlink" title="得到flag"></a>得到flag</h4><p><img src="test.png" alt="PNG文件头"></p>
<h1 id="小结"><a href="#小结" class="headerlink" title="小结"></a>小结</h1><p>等写完程序后才发现自己是不是傻。既然可以修改图片的宽高，我为什么还要写个程序来跑？直接随便修改一个值不就好了，改大点就行。反正windows下的看图工具能识别。最后试了一下，果然是可以的。</p>

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